Measurement of the shape and thickness of biological tissue layers can provide useful diagnostic information in many applications. For example, an abnormally thick epicardial adipose tissue layer may be predictive of significant coronary artery disease, and a noticeable thickening of breast tissue can be caused by inflammatory cancer.
In ophthalmology, retinal thickness can be abnormally large in cases of retinal edema or traction by membranes in the vitreous humour. On the other hand, the retina may appear thin in cases of atrophic degeneration, chorioretinitis, or trauma to the retina. In particular, age related macular-degeneration (AMD) is a condition that results in a loss of vision in the centre of the macular because of damage to the retina. Early signs of AMD such as drusen, being cellular debris accumulating between the retina and the choroid, can be detected using the shape of the surface of the retinal pigment epithelium (RPE) layer. A more advanced form of dry AMD, geographic atrophy (GA), can be detected by measuring the thickness of RPE layer.
Optical coherence tomography (OCT) is a medical imaging technique based on low-coherence interferometry employing near-infrared light. OCT produces 3D images with micrometer resolution from within optical scattering media and is widely used in ophthalmology due to the translucent nature of human eye. In Frequency Domain OCT (FD-OCT), the interferometric signal between reference light and the back-scattered light from a sample point in the eye is collected by a line camera. This collected data contains the spectral information of the backscattered signal. After converting the back-scattered light from wavelength to the frequency domain, a one-dimensional Fourier transform is taken to obtain a 1-D spatial distribution of the object scattering potential (A-scan). Laterally scanning the sample beam over a series of adjacent A-scans creates a 2-D tomogram, called a B-scan. Volumes are acquired by further scanning the sample beam in another direction to collect a series of B-scans that covers the 3-D volume of interest.
As the RPE layer is darkly pigmented, it absorbs OCT light and only a weak signal is reflected back to the OCT instrument. In order to identify the retinal pigment epithelium (RPE) layer from a set of 3-D OCT data, one segmentation technique uses the highly scattering property of the RPE, where the second major peak in reflectivity in an A-scan is identified as the RPE. Some recent segmentation algorithms extract the RPE layer in an OCT B-scan by looking for reflectivity peaks. However, it is well known that the intensity of the back-scattered signal alone is not sufficient to be able to distinguish tissue types.
In recent years, functional extensions of OCT have been shown to provide additional contrast by recording not only reflectivity profiles but also information about flow velocity (Doppler OCT) or polarization properties (Polarization-Sensitive OCT, “PS-OCT”) of the tissues. For example, in the human retina, birefringent, polarization-preserving, and depolarizing structures can be distinguished by PS-OCT. In the last few years, segmentation of RPE was demonstrated using PS-OCT based on the depolarizing character of the RPE layer. In one method, the position of the Bruch's membrane is estimated using a fixed distance from the inner limiting membrane (ILM), where the location of the ILM is detected in a reflectivity OCT B-scan. Within a certain tolerance, depolarizing tissues below the estimated Bruch's membrane location are classified as belonging to the choroid, and other depolarizing tissues in the vicinity of the estimated Bruch's membrane are classified as RPE. The RPE thickness is then calculated and an en face map, showing the RPE thickness of all B-scans, can be used to diagnose geographic atrophy (GA). Because of the assumed fixed distance from the ILM to the Bruch's membrane, this method fails in cases when the ILM is deformed or when large errors occur in ILM detection.
Another method uses a simple smooth surface fitted to a plurality of detected RPE tissues as the estimate of the location of the Bruch's membrane. Due to depolarizing tissues that are not RPE floating in the retinal area, the fitted smooth surface tends to deviate from the Bruch's membrane location and provides an inaccurate baseline for RPE thickness calculation. Furthermore, for eyes with drusen, where build-up of fatty protein between the RPE and the choroid creates ripples and folds in the RPE layer, a surface fitted to detected RPE tissues is no longer a good estimate of the Bruch's membrane location. In advanced cases of AMD, larger and more numerous drusen in the RPE as well as large geographic atrophy (GA) become challenging for the above segmentation methods. The detection of drusen and GA often relies on the accurate segmentation of the assumed healthy RPE location, the RPE location when there is no retinal disease; the large deformation caused by large drusen or GA regions creates unwanted artefacts and noise in OCT or PS-OCT images and automatic segmentation of such images often fails to produce good results.
Therefore, a new robust method is needed to estimate the location of a specific tissue layer.